1. Field of the Invention
The present invention relates to a semiconductor device that has a high withstand voltage and enables an employment of a large diameter substrate, and a method of manufacturing this sort of semiconductor device. The present invention also relates to isolating high-voltage withstanding elements from each other in the semiconductor device.
2. Description of the Related Arts
FIG. 4 shows a planar semiconductor device. When a backward voltage is applied to a PN junction of the semiconductor device, an electric field concentrates more to a curved portion than to a flat portion. An avalanche breakdown voltage at the curved portion is, therefore, lower than that at the flat portion. To cope with this, a high-voltage withstanding structure such as a mesa structure shown in FIGS. 5(a) and 5(b) or a guard ring structure shown in FIG. 6 is employed when a high withstand voltage of 600 V or over is needed.
The mesa structure shown in FIGS. 5(a) and 5(b) is formed by obliquely polishing or etching side faces of an element. The mesa structure has a flat PN junction, and therefore, an electric field does not locally concentrate, thereby realizing a high withstand voltage. In view of an electric field at an edge of the PN junction, an inverted mesa structure shown in FIG. 5(b) is superior to a normal mesa structure of FIG. 5(a). The structure of FIG. 5(b) causes a weaker electric field at the edge of the PN junction to realize a higher withstand voltage.
Conventionally, the oblique side faces of the inverted mesa structure of FIG. 5(b) are processed in a later stage. This means that an etching quantity of the mesa will be increased as the thickness of a semiconductor substrate is increased. If a large diameter substrate is employed to improve an integration of elements, the thickness of the substrate is inevitably large, thus increasing the etching quantity. Then, even if the large diameter substrate is employed, the integration of elements will be hindered.
The guard ring structure of FIG. 6 having a PN junction with edges thereof protected with oxide films is easy to produce. If a high withstand voltage is required in the guard ring structure, however, the number of guard rings must be increased and a depletion layer horizontally extended to relax an electric field at a curved portion of the PN junction. This requires an extended area to increase the size of the structure. The curved portion cannot be eliminated from the structure, so that the withstand voltage thereof will be lower than that with a flat junction.
The inverted mesa structure of FIG. 5(b) is called a bevel structure. In this structure, elements are formed and then isolated from each other by chemical etching carried out on the reverse side of the structure. When a large diameter wafer is employed, the wafer is thick to increase an etching quantity. This reduces an effective area of each element. The bevel structure is, therefore, not suitable for manufacturing semiconductor devices from the large diameter wafer. According to the bevel structure, elements and wires electrodes are firstly formed, and the elements are then isolated from each other by chemical etching, so that isolated or etched side faces of the elements are exposed and unable to be coated with protective films such as thermal oxidation films.